There is no difference between cardboard box, and blue box lapua brass. If your loaded rounds measure at .269 and the chamber speck is .2705, that is giving you .0015 clearence. Some of the old benchrest shooters use to shoot what they called fitted necks wich is about what you got, they would fire the case then just seat the bullet without sizing. The problem is when you do this it can lead to unexplained flyers, and other problems. I prefer to have a bit more tolarence. Jack Neary (One of the top point blank benchrest shooters in the world.) has a saying and it is "Go thin to win." Jack runs .003 to .0035 clearence on his bechrest gun. With that said you need to turn your brass a bit.